Field
The following description relates generally to telecommunication systems, and more particularly to managing access to a transmission medium in a wireless environment.
Background
The deployment of wireless local area networks (WLANs) in the home, the office, and various public facilities has experienced tremendous growth over the past several years. Typically, a WLAN is implemented with a wireless access point (AP) that connects a number of wireless stations (STAs) in a specific locality (e.g., home, office, or public facility) to a wide area network (WAN), such as the Internet or the like. In larger deployments, the locality may be divided into a number of cells with each cell having its own wireless AP. Each wireless AP is configured to connect the STAs in its cell to the wired network. Alternatively, the WLAN may be implemented as a mesh network where STAs operate in an ad-hoc mode to provide multi-hop routing for traffic though the WLAN.
STAs that operate in an IEEE 802.11 compliant WLAN use carrier sensing to access the transmission medium. Carrier sensing is a media access control (MAC) protocol in which a STA verifies the absence of traffic in the shared medium before transmitting. One type of carrier sensing supported by IEEE 802 is physical carrier sensing which samples the energy level in the medium and transmits only if the energy level is below a carrier sensing threshold. Another type of carrier sensing supported by IEEE 802.11 is virtual carrier sensing which uses a handshake protocol between the transmitting and receiving STAs to effectively reserve the medium. Virtual carrier sensing is often used to avoid the hidden node problem that is often encountered in mesh networks and large deployments with unknown topologies. However, in deployments with a single AP serving a smaller locality (or a cell within a locality), interference between STAs can often be mitigated through channel assignments and scheduling. In these deployments, physical carrier sensing may be used to avoid the additional overhead requirements of virtual carrier sensing.
Although physical carrier sensing may be adequate to mitigate interference within a cell or locality, interference between two APs operating in nearby localities or in neighboring cells can be problematic. Generally, the APs communicate with one another to optimize transmission power to avoid interference. However, a power imbalance can still occur where a first AP transmits at a higher power level than a second other AP. In that case, it possible that high power transmissions by the first AP may be detected by the second AP, but the lower power transmissions by second AP may not be detected the first AP. As a result, the first AP will transmit even though the second AP is transmitting simultaneously because it cannot detect the lower power transmissions by the second AP. However, the second AP will back off from transmitting when the first AP is transmitting. This creates an unfairness which provides the first AP with effectively exclusive access to the transmission medium.
Conventional WLANs have addressed this issue by increasing the carrier sensing threshold of the second AP to a level where it can no longer detect the transmissions by the first AP. This approach, however, tends to cause packet collisions at the STAs served by the second AP. Accordingly, there is a need in the art for managing access to the medium between APs using a carrier sensing protocol.